1. Field of the Invention
This invention relates to color printing such as used in electrophotography and more specifically to a method and apparatus of forming a multicolor image on the same frame or area of a recording member, so that it may be transferred or otherwise utilized in a single step.
2. Description of the Prior Art
In the prior art as represented by International Publication Number WO 90/01 730, images can be formed consecutively on the same image frame of a photoconductor without fixing the first image by the use of successive steps of charging, exposing and developing and then repeating such steps for each color to be created upon that image frame. The resulting multicolor image is transferred to a receiving sheet in one step thereby increasing the productivity of the printer or copier.
The exposure steps as taught in this publication are carried out using separate LED (light-emitting diode) printheads for each color to be formed. Typically, these printheads have a single row of several thousand LED's arranged at say 300 dots per inch across the photoconductor and are controllably illuminated to modulate an electrostatic charge on the photoconductor to form images on a dot-by-dot or pixel-by-pixel basis.
A problem with the use of plural LED printheads is the alignment of both, relative to each other so that image recorded colored pixels are appropriately formed in a desired pattern, i.e., that they are registered. In view of the small spacing between LED's on a printhead, incorrect location of the printhead will cause images to be misregistered thereby affecting color and quality of the resulting image. To this end, expensive mechanical means may be devised to ensure that one printhead is accurately located relative to another but this raises considerable costs to the manufacture of a printer apparatus.
It is therefore an object of the invention to provide improved means for ensuring registration of the printheads used in a color printer apparatus having two or more printheads.
It is a further objection of the invention to provide improved means for registering images vis-a-vis the printing of same on a copy sheet.